1. Field of the Invention
The present invention relates to art of making electronic devices and more specifically relates to methods of encapsulating microelectronic assemblies.
2. Description of the Related Art
When making microelectronic assemblies, such as semiconductor chip packages, it has been found desirable to provide an encapsulant between and/or around the different components of the microelectronic assemblies. It is theorized that using an encapsulant minimizes strain and stress on the electrical connections between the microelectronic assemblies and supporting circuitized substrates during operation of the assemblies. An encapsulant may also seal the components of the microelectronic assemblies against corrosion, as well as insure intimate contact between the encapsulant and the other elements of the microelectronic assembly.
Microelectronic assemblies such as semiconductor chip packages are typically encapsulated using curable liquid compositions. Such liquid compositions frequently cure upon exposure to elevated temperatures or light.
When encapsulating microelectronic assemblies, the liquid composition is generally dispensed or injected around or into the assembly. In many instances, a fixture is placed around the microelectronic assembly prior to dispensing the liquid composition. The fixture generally serves as a dam that ensures that the dispensed liquid composition flows around and between the components of the microelectronic assembly. After the liquid composition has been dispensed, the fixture may be placed in an oven to cure the liquid composition. Once the composition is cured, the fixture is removed from the oven and the one or more microelectronic assemblies are removed from the fixture. In order to improve the rate at which the microelectronic assemblies are manufactured, it may be desirable to remove encapsulated assemblies from the fixture before the curing step in order to increase the number of fixtures available for use on other batches of microelectronic assemblies.
Methods of using curable liquid compositions to encapsulate microelectronic assemblies are described in certain preferred embodiments of commonly assigned U.S. Pat. No. 5,659,652; U.S. Pat. No. 5,766,987; U.S. Pat. No. 5,776,796; U.S. patent application Ser. No. 09/067,698, filed Apr. 28, 1998; and U.S. patent application Ser. No. 08/610,610, filed Mar. 7, 1997, the disclosures of which are hereby incorporated by reference herein. Certain preferred embodiments of the above-identified patents and patent applications describe methods of making microelectronic assemblies including a semiconductor chip and a flexible dielectric sheet having terminals and flexible leads connected to the terminals. During assembly, contacts on the chip are electrically interconnected to terminals on the dielectric sheet by attaching tip ends of the leads to the chip contacts. In certain preferred embodiments, the electrical interconnection is made using flexible, electrically conductive leads having first or tip ends attached to chip contacts and second ends connected with terminals of the dielectric sheet. The assembly may also include a compliant layer such as a cured elastomer or gel disposed between the dielectric sheet and the chip to mechanically decouple the terminals of the sheet from the chip. As disclosed in certain preferred embodiments of the aforementioned patents and patent applications, such compliant layers can be made by providing a porous layer, such as a plurality of compliant pads, between the chip and the dielectric sheet, electrically connecting the terminals to contacts on the chip using conductive leads and then encapsulating the resulting assembly with a curable liquid composition so that the liquid composition penetrates through the porous layer and around the leads. Upon curing, the composition provides a compliant layer between the chip and the dielectric sheet. When making microelectronic assemblies of this nature, it is desirable to ensure that the encapsulant completely fills the porous layer, to provide a substantially void-free compliant layer in the final assembly.
There are many preferred methods for introducing and curing a curable liquid composition in microelectronic assemblies. For example, in certain preferred embodiments of the above-identified ""987 patent, one or more cover layers may be applied over the top and bottom surfaces of one or more microelectronic assemblies, each assembly including at least one semiconductor chip electrically interconnected to at least a portion of a dielectric sheet. Typically, several microelectronic assemblies are provided in a side-by-side arrangement so that a single top cover layer overlies the top surfaces of the chips and a bottom cover layer underlies the bottom surface of the dielectric sheet. The top and bottom cover layers and the microelectronic assemblies may then be placed in a fixture. The top and bottom cover layers define a space therebetween. A vacuum is desirably drawn in the space between the top and bottom cover layers to remove air from the enclosed space and to ensure that the outside pressure is greater than the pressure between the cover layers. The fixture is then tilted so that a curable liquid encapsulant stored in a pocket of the fixture is able to flow into the space between the cover layers. The curable liquid composition is then cured while the microelectronic assemblies and cover layers remain in place in the fixture.
As described in certain preferred embodiments of commonly assigned U.S. Pat. No. 5,659,952, the disclosure of which is hereby incorporated by reference herein, and the aforementioned U.S. Pat. No. 5,776,796, the encapsulant may be applied using a needle or syringe that is positioned around the periphery of each microelectronic assembly. The needle, desirably connected to an encapsulant dispenser, is moved around the peripheries of the individual microelectronic assemblies so that the encapsulant flows into the space between each chip and a dielectric sheet. A cover layer may be used to close any bond windows in the dielectric sheet during this process. In other preferred embodiments for encapsulating microelectronic assemblies, as described in U.S. patent application Ser. No. 08/975,590 filed on Nov. 20, 1997, the disclosure of which is hereby incorporated by reference herein, the encapsulant dispensing operation is conducted inside a chamber, while the microelectronic assembly is under vacuum. When the vacuum is released, and the chamber is brought to atmospheric or superatmospheric pressure, the pressure forces the encapsulant into the porous layer between the chip and the dielectric sheet. Further, as described in U.S. patent application Ser. No. 09/012,590, filed Jan. 23, 1998, the disclosure of which is also incorporated by reference herein, it is convenient to use a frame to hold the dielectric sheet during the assembly procedures. The frame enables an operator to handle the dielectric sheet without directly contacting the portions of the sheet that comprise part of the final assembly. For example, while the dielectric sheet is mounted on the frame, an array of compliant pads used to form the porous layer may be applied to the dielectric sheet, the chips may be attached to the pads, and the terminals of the dielectric sheet may be electrically interconnected with the chips using flexible leads.
In spite of the above-mentioned advances in the art, there remains a need for improved methods for encapsulating microelectronic assemblies.
The present invention provides methods of encapsulating one or more microelectronic assemblies and is specifically directed to encapsulating microelectronic assemblies mounted to a substrate, such as a printed circuit board. In one preferred embodiment, the method includes providing a substrate having a top surface and securing at least one microelectronic assembly over the top surface of a substrate. A dam is provided over the top surface of the substrate, the dam surrounding the at least one microelectronic assembly mounted to the substrate. The substrate preferably includes a printed circuit board having one or more conductive pads on a top surface thereof, the conductive pads being used to form an electrical connection between the substrate and the microelectronic assembly.
Preferred microelectronic assemblies may include a semiconductor package including a semiconductor chip, a dielectric sheet having terminals on a bottom surface thereof, and leads electrically connecting chip contacts to the terminals of the dielectric sheet. The above-mentioned semiconductor package may include a porous layer disposed between the contact bearing face of the chip and the dielectric sheet. The porous layer may comprise a plurality of compliant pads having voids between the pads. Each package may also include a compliant material, such as a cured encapsulant, between the compliant pads and between the chip and the dielectric sheet.
A dam is preferably provided atop the first surface of the substrate and desirably surrounds the one or more microelectronic assemblies secured to the substrate. The dam may be a separate structure that is adhered to the top surface of the substrate. The dam may also be formed atop the substrate by dispensing a material into a mold or frame positioned atop the substrate and then curing the material dispensed into the mold to form the dam. The dam may also be integrally formed with the substrate, whereby the substrate and the dam are a single piece. The dam preferably includes any material capable of retaining a curable liquid encapsulant within a bounded area atop a substrate or circuit board until the liquid material can be cured into a solid. In highly preferred embodiments, the dam preferably includes an epoxy material. The curable liquid encapsulant is preferably cured using heat or light.
The method of encapsulating one or more microelectronic assemblies also desirably includes providing a cover layer over the dam and over the at least one microelectronic assembly mounted to the substrate. After the cover layer has been placed over the dam, the substrate, dam and cover layer desirably define an enclosed space that encompasses the at least one microelectronic assembly secured to the substrate. A curable liquid encapsulant may then be injected into the enclosed space and around the at least one microelectronic assembly. In certain preferred embodiments, the enclosed space may be evacuated, such as applying vacuum to remove any air in the enclosed space, prior to the step of injecting the liquid encapsulant. It is theorized that evacuating the enclosed space before introducing the curable encapsulant into the enclosed space minimizes the formation of air pockets or voids in the cured encapsulant layer.
Injecting the curable liquid encapsulant may include providing a dispenser, such as a needle in communication with the enclosed space and passing the liquid encapsulant into the enclosed space through the needle. The air in the enclosed space may be evacuated through the needle before the curable liquid encapsulant is introduced through the needle. The cover layer may include a hole between the external atmosphere and the enclosed space and the hollow needle may be advanced through the hole prior to passing the liquid encapsulant into the enclosed space. The above-mentioned step of evacuating the enclosed space may be performed by applying a vacuum through the hole in the cover layer or through the needle placed into the hole. The cover layer is preferably resilient so as to sealingly engage the needle at the hole when the needle is inserted into the hole.
In certain embodiments, the cover layer engages the back surface of the semiconductor chip so as to prevent any of the liquid encapsulant from coming into contact with the back surface of the chip when the encapsulant is introduced into the enclosed space. In other embodiments, a plate may be positioned over the back surface of the chips to prevent the cover layer from moving away from the chips as the encapsulant is introduced. Preventing contamination of the back surface of the chips is highly desirable. For example, if the back surface of a chip is free of contaminants, a thermally conductive layer and/or a heat sink, may be placed in communication with the back surface for removing heat from the one or more microelectronic assemblies. In other preferred embodiments, the cover layer is not in contact with the back surface of the microelectronic element so that the introduced curable liquid encapsulant may flow between the back surface of the microelectronic assembly and the cover layer. When the liquid encapsulant is cured, an insulating layer of encapsulant is formed atop the back surfaces of the one or more microelectronic assemblies.
In further embodiments, the microelectronic assembly is attached to the substrate by first securing a dielectric sheet having conductive terminals atop the substrate so that the terminals of the dielectric sheet are electrically interconnected with conductive pads on the substrate. Next, a semiconductor chip having a front contact bearing face is placed atop the dielectric sheet. The chips are electrically connected with the dielectric sheet using leads. A plurality of compliant pads may be provided between the chip and the dielectric sheet.
One or more of the microelectronic assemblies may have a porous layer between the microelectronic element and the dielectric sheet. The porous layer may include compliant pads or nubbins between the microelectronic element and the dielectric sheet. Preferably, the compliant pads are applied to the dielectric sheet before the microelectronic element is connected to the sheet. During the abovementioned step of injecting the liquid encapsulant, the liquid encapsulant preferably flows between the microelectronic element and the dielectric sheet and fills the voids between the compliant pads.
One or more platens may be placed atop the cover layer and/or below the substrate during the step of injecting the encapsulant. The platens may be held in place during the injecting the encapsulant step to control movement of the cover layer, the microelectronic assemblies and/or the substrate. In other preferred embodiments, one of the platens may be allowed to move in a controlled fashion away from one of the other platens during the injecting step so as to allow one or more leads interconnecting the microelectronic element with the dielectric sheet to move or flex during the injecting step.
In still further embodiments, the cover layer may be formed from a thermally conductive material such as a metallic sheet. After the encapsulant has been introduced and cured, a portion of the metallic sheet may remain attached to one or more of the microelectronic assemblies so that the metallic sheet may transfer heat from the assembly during operation thereof. In further embodiments, the thermally conductive layer may include one or more bores formed therein that face the substrate when the thermally conductive layer is provided atop the microelectronic assembly and the dam. Preferably, the bores are adapted for receiving the liquid encapsulant as it is injected between the cover layer and the substrate. The liquid encapsulant flows into the bores before the encapsulant is cured and provides protrusions or fingers comprising cured encapsulant that are effective for reliably adhering the conductive sheet to the cured encapsulant. Thus, the cured encapsulant in the bores provides additional gripping elements extending into the thermally conductive sheet or cover layer for providing enhanced adherence of the cover layer to the encapsulant.
Another preferred embodiment of the present invention provides a method of making a microelectronic assembly including attaching at least one microelectronic assembly to a substrate, providing a dam atop the substrate, wherein the dam surrounds the at least one microelectronic assembly and applying a cover layer over the dam and over the at least one microelectronic assembly attached to the substrate so as to define an enclosed space encompassing the at least one microelectronic assembly secured to the substrate. A curable liquid encapsulant is then injected into the enclosed space and around at least one microelectronic assembly and the encapsulant is cured. During the curing step, the dam and the cover layer confine the encapsulant so as to prevent the encapsulant from contacting a rear surface of the at least one microelectronic assembly attached to the substrate. In certain embodiments, the method also includes the step of evacuating the enclosed space between the cover layer and the dam and around the at least one microelectronic assembly prior to the step of injecting the liquid encapsulant. The above-mentioned curing steps may be performed at an elevated temperature or by applying light.